Method for the modification of wood

ABSTRACT

The invention is directed to a process for the continuous acetylation of wood elements. The process particularly combines a batchwise impregnation step, with a continuous reaction step. In order to realize this, a collection step is built-in, so as to allow batches of impregnated wood elements to be fed into a reactor in a continuous manner. Very high acetylation contents can be obtained, at a level that had not been achievable before in a continuous and non-catalyzed acetylation process.

FIELD OF THE INVENTION

The present invention relates to chemical modification of a wood basedmaterial (lignocellulosic material) by means of acetylation.Particularly, the present invention is in the field of acetylation ofwood elements and provides a process for continuous acetylation thereof.

BACKGROUND OF THE INVENTION

In order to produce wood-based materials with a long service life, ithas been known to chemically modify the wood and in particular toacetylate the wood. Thereby materials with improved material properties,e.g. dimensional stability, hardness, durability, etc., are obtained.

In the art, it is known to use batch processes, i.e. stop/startprocesses for the acetylation of wood particles. Such processes,however, often lead to poor product uniformity as the properties differsignificantly from batch to batch and no two batches can ever beconsidered as identical.

Some references address the option to conduct wood acetylation in acontinuous process. Thus, e.g., EP 0746570A1 discloses a process for theacetylation of lignocellulosic materials (LM) comprising a first stepwherein the LM is brought into intimate contact with an acetylatingagent comprising acetic anhydride as the major component at atemperature from 80° C. to 140° C. and bringing the acetylated LM fromthe first step into contact with a heated gas inert under the reactionconditions in a stripper at a temperature above 140° C. A comparableprocess is described in EP 650 998.

To the extent that these references would enable conducting a continuousprocess, this is on the basis of a plug screw feeder type ofimpregnation in series with continuous further processing (e.g.reaction). This is described for wood elements of relatively smalldimensions, e.g., fibres, small particles, etc. When used for woodelements of larger dimensions, such as chips or strands, the plug screwwill normally lead to a product wherein the wood dimensions have becomesmaller (crushed chips, crushed strands, broken fibres, smallerparticles etc.). Particularly, if chips or strands are used, no pluggingeffect can be achieved to separate between zones with differentpressures and/or atmospheres without destruction/crushing of the chipsor strands. It is desired to provide a process for the continuousacetylation of wood particles, and particularly of wood chips orstrands, wherein the particles are not damage in the way as done whenusing a plug screw feeder.

Moreover, in the acetylation of wood elements, particularly of woodchips, a technical challenge in providing a suitable continuous process,is to combine the advantages of a continuous process, with the desiredresult of a sufficiently high degree and uniformity of acetylation.

It should be noted that in determining wood acetylation degrees, twodifferent approaches are taken in the field. One is based on WPG (WeightPercentage Gain). WPG compares a sample before and after acetylationtreatment, and as a result any substances added (and any residues stillpresent in the wood) increase the value. WPG is explained in thefollowing formula: WPG=(M_(increase)/M_(sample before reaction))×100%.Herein M stands for mass, andM_(increase)=M_(sample after reaction)−M_(sample before reaction)).

The other approach, is to actually measure the acetyl content (AC). Thisis given as AC=(M_(acetyls)/M_(sample after reaction))×100%. TypicallyHPLC (high-pressure liquid chromatography) can be used to quantify theacetate ion concentrations resulting from the saponification of acetylgroups from the wood. From this the overall mass after acetylation canbe taken as M_(acetyls).

The different results for WPG and AC can be explained with reference tothe following theoretical example: a sample of, e.g., 1 g of wood isacetylated and after the reaction has a mass of 1.25 g. Thus M_(acetyls)is 0.25 g. The resulting WPG is: (1.25-1.00)/1.00*100%=25%. Calculatedas acetyl content, AC is =(1.25-1.00)/1.25*100%=20%.

Hence, care should be taken not to directly compare degrees ofacetylation expressed in WPG with degrees of acetylation expressed inAC. In the present description AC values are elected to identify thedegree of acetylation.

The present invention aims to provide a continuous process for theacetylation of wood elements, thereby securing the concomitantadvantages of consistency in quality (such as acetyl levels, andpreferably also residual acetic acid and residual acetic anhydridelevel) and cost over existing batch processes. Particularly, theinvention also aims to provide a continuous acetylation process thatenables the production of all wood elements having a relatively highdegree of acetylation.

It is notoriously difficult to obtain acetylation degrees of AC higherthan 20% for wood elements. In the art, such degrees are only obtainedvia catalyzed processes. In catalyzed acetylation processes, a catalystfor the acetylation reaction is added to the wood. These are frequentlytoxic organic substances, such as pyridine, but also sodium bicarbonate,potassium acetate, and other salts, particularly acetate salts areknown. The invention aims at providing a non-catalyzed process, thusavoiding the drawback of having residues of additional foreignsubstances (viz. the catalyst) remain in the wood.

SUMMARY OF THE INVENTION

In order to better address one or more of the foregoing desires, theinvention presents, in one aspect, a process for the continuousacetylation of wood elements comprising the following steps:

(a) impregnating a plurality of batches of wood elements with anacetylation fluid comprising acetic anhydride and/or acetic acid so asto provide a plurality of batches of impregnated wood elements;

(b) collecting said plurality of batches of impregnated wood elements insuch a manner as to provide a continuous feed of impregnated woodelements to a Reaction Chamber;

(c) subjecting a continuous flow of impregnated wood elements toacetylation reaction conditions in said Reaction Chamber so as toprovide a continuous flow of acetylated wood elements exiting saidReaction Chamber.

In another aspect, the invention provides a plant for the acetylation ofwood elements, said plant comprising

(i) at least one Impregnation Chamber,

(ii) a Collection Chamber having an inlet in connection with an outletof the at least one Impregnation Chamber, said connection being such asto allow wood elements to flow from the Impregnation Chamber to theCollection Chamber;

(iii) a Reaction Chamber having an inlet in connection with the outletof the Collection Chamber, said connection being such as to allow woodelements to flow from for the Collection Chamber to the ReactionChamber, wherein the Reaction Chamber is designed so as to provide acontinuous transport of wood elements from the inlet to an outletthereof.

In yet another aspect, the invention presents acetylated wood elements,obtainable by a non-catalyzed acetylation process, and preferably chips,strands, or particles, wherein the acetylated wood elements have anacetyl content (AC) of at least 20.5%, preferably at least 21%, andparticularly of AC 21% to 26%.

In a further aspect, the invention provides a process for the continuousacetylation of wood elements comprising the following steps:

(a) providing wood elements impregnated with an acetylation fluidcomprising acetic anhydride and/or acetic acid so as to have a RetainedWeight Gain, as defined below, of at least 20%;

(b) introducing said impregnated wood elements into a Reaction Chamber;

(c) subjecting a continuous flow of impregnated wood elements toacetylation reaction conditions in said Reaction Chamber so as toprovide a continuous flow of acetylated wood elements exiting saidReaction Chamber.

DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing depicting a flow scheme of a process andplant according to the invention. The reference signs in the drawinghave the following meaning:

(1) Incoming wood element stream, in this drawing this incoming woodelement stream is received in a dosing hopper-screw combination;

(2) Impregnation Chamber that can be closed off from the surroundingatmosphere;

(3) Vacuum connection;

(4) Acetylation filling and pressurizing connection;

(5) Collection Chamber;

(5a) vessel to collect and store the impregnated wood elements;

(5b) outlet from the Collection Chamber allowing impregnated woodelements to be fed into the Reaction Chamber;

(5c) drain for separation of excess liquid;

(6) Continuous Reaction Chamber according to first-in-first-outprinciple;

(7) Outgoing stream of acetylated wood elements.

DETAILED DESCRIPTION OF THE INVENTION

The invention, in a broad sense, is based on the judicious recognitionthat an optimal continuous acetylation process for wood elements, can berealized starting from a batchwise impregnation with acetylation fluid.

In discussing this process, the following basic definitions are to betaken into account.

An Impregnation Chamber is a vessel, or an otherwise suitable containerto hold wood elements and bring these into contact with an acetylationfluid. Preferably, the Impregnation Chamber can be closed off from thesurrounding atmosphere, and be operated under reduced as well asincreased pressure, preferably under vacuum. In general, theImpregnation Chamber will be a vessel that can be filled with solids,evacuated, filled with liquids, pressurized. Such vessels are known tothe skilled person. From the vessel, a liquid including impregnatedchips can be discharged without damaging the impregnated chips. To thisend, preferably, a vessel is used having a relatively large dischargeopening (e.g. 200 to 500 mm), preferably a conic vessel allowing anangle of repose of 10° to 25°, and a relatively low surface roughness(preferably 0.1 μm to 5 μm, more preferably 0.5 μm to 2 μm).

The Impregnation preferably is suitable for conducting a Bethel type ofwood impregnation process. In the Bethel process, the wood is placed ina vacuum chamber and vacuum is applied to draw air from the wood. Activeingredients (in the present invention: an acetylation fluid) is thenadded to the chamber under vacuum. After filling the chamber with liquida pressure generally up to 250 pounds per square inch (psi) can beapplied, preferably 150 psi to 200 psi. The pressure is removed so thatthe wood is again subject to atmospheric pressure. This type of processis preferred, since it should normally result in a maximum impregnationload, which is believed to have a direct relation to a desired maximumacetyl level.

However, in the event that maximum acetylation fluid uptake afterimpregnation is not a primary goal, more economical impregnationprocesses can also be used. Examples thereof, known to the personskilled in the art of wood impregnation, are the so-called Lowry andRueping processes. These processes require no initial vacuum. Instead,the impregnation fluid is forced deep into the wood under pressure. Thecompressed gas inside the wood then expands when the pressure isreleased, causing any excess preservative to be forced out of the wood.

Also methods different from the Lowry or Rueping processes can be used,such as spraying methods. The latter is typically used in fullycontinuous wood acetylation processes, which have as one of theirdrawbacks that relatively low acetyl levels are obtained. One of theadvantages of the present invention is the freedom to choose theimpregnation conditions irrespective of the reaction conditions and viceversa.

A Collection Chamber is a vessel, a funnel, or otherwise any devicesuitable to collect and store impregnated wood elements. If employed,the Collection Chamber will have an inlet allowing impregnated woodelements to be received from one or more Impregnation Chambers. Ifemployed, the Collection Chamber will have an outlet allowingimpregnated wood elements to be fed into a Reaction Chamber. TheCollection Chamber preferably has a funnel-type geometry. This provides,in a technologically simple manner, a suitable inlet, a suitable placeto hold a buffer volume of impregnated wood elements, and an outletallowing a controlled amount of impregnated wood elements to be fed intoa Reaction Chamber. The Collection Chamber functions, inter alia, as areceiving vessel to smoothen any peak loads from unloading theImpregnation Chamber towards the Reaction Chamber; further to allowlonger submerged contact time between chips and acetylation fluidresulting in further swelling of the wood chips material and thusincreased acetylation fluid uptake. Additionally in the CollectionChamber (also referred to as a receiving/buffering vessel) excess liquidmay be separated from the ongoing impregnated wood elements that go onto the continuous reactor. Also, the Collection Chamber can be used tointroduce additional temperature control.

A Reaction Chamber is a vessel, a reactor, or otherwise any devicesuitable to subject wood elements, impregnated with acetylation fluid,to reaction conditions that bring about acetylation of the wood. In theinvention the Reaction Chamber, as opposed to reaction chambers used inbatch acetylation processes, is operated as continuous reactor. In aninteresting embodiment, the Reaction Chamber is operated with two ormore zones wherein different temperatures are applied. This is asuitable measure to optimize the application of heat to the woodelements in the course of the acetylation reaction. Preferably two tofive zones are applied. The skilled person will be able to determine thedesired heating regimen, i.e. the number of zones, the temperaturesapplied therein, and the effective residence time in each of thesezones. It will be understood that the precise optimization depends oncircumstances such as the type and shape of wood to be acetylated andthe specific apparatus chosen for acetylation.

Suitable reactors include, but are not limited to, liquid or gas-phasereactors known in the art of wood acetylation. A gas-phase reactor ispreferred. This type of reactor allows subjecting the impregnated woodelements at higher temperatures, whilst at relatively low pressures. Asa result, the residence time in a gas-phase type reactor can generallybe longer than in e a liquid-type reactor, which is beneficial to theacetylation levels. Also, in a gas-phase reactor the wood elements arenot prone to washing out of acetic acid. In case of a liquid-phasereactor (i.e., without separating of excess liquid from the impregnatedparticles) washing out of this acetic acid would result in lowering theacetic anhydride concentration in the surrounding acetylation fluid.Also resins, as a desirable component of wood, are better retained in agas-phase type reactor than in a liquid type reactor.

In order to be suitable for continuous operation, the Reaction Chamberhas an inlet connected to the Collection Chamber or, if the latter isdispensed with, a plurality of Impregnation Chambers. The ReactionChamber itself is designed in such a way as to allow the input ofimpregnated wood elements, and the output of acetylated wood elements,to be continuous. This can be realised in various ways, controlled bygravitation, by mechanical forces, or both. Preferably, the woodelements are carried through following the principle of plug flow, whichimplies a “first-in-first-out” principle.

The Reaction Chamber may comprise a vertically arranged plug flowreactor through which the wood elements pass downwardly through anacetylation fluid. An example of simple gravitational flow is a ReactionChamber that is tilted in the sense that the inlet is positioned higherthan the outlet. As a result thereof, wood elements will flow, bygravitation, from the inlet to the outlet, whilst being subjected toacetylation conditions. An example of mechanical force is a ReactionChamber comprising a screw conveyor serving to transport wood elementsfrom the inlet to the outlet. In one embodiment, the transportationforces will be provided by a combination of mechanical forces exerted bya screw and gravitational forces provided by having an inlet at higherlevel than an outlet. Most preferably, a screw conveyor is employedhaving a substantially horizontal screw, as a result of which thetransportation is fully controlled by the mechanically operated screw,and not affected by gravitation. Both single and multiple screwconveyors can be used.

The process of the invention is particularly suitable for obtaining woodelements acetylated so as to have relative high acetyl levels,preferably higher than 18%, more preferably at least 20.5%, morepreferably higher than 21%, still more preferably of from 21% to 26%,all percentages expressed as AC values.

Without wishing to be bound by theory, the present inventors believethat the possibility to obtain such high levels in a continuousacetylation process, is determined initially by the judicious selectionof a batchwise impregnation, the process conditions of which can betuned independently of the actual acetylation step.

As mentioned above, the impregnation may be conducted so as to result ina maximum impregnation load (which, in theory, would mean that the woodelements are fully saturated with impregnation fluid), but also bymethods not leading to the maximum.

Particularly, the present inventors believe that the batchwiseimpregnation as described above is capable of resulting in wood elementsthat are impregnated not only in easily accessible voids, but also inspots that are more difficult to access. The latter spots aredistinguished by the fact that, once impregnated, they retain theacetylating liquid to a great extent. The nature of this retaining maybe based on chemical interaction with the wood as well as on possiblephysical interaction with the wood matrix. Whether or not this type ofimpregnation has occurred can be determined with reference to theRetained Weight Gain (RWG).

The RWG is determined as follows:

reference value is the mass of a volume of wood elements contained in aglass cylinder, of known empty weight, internal diameter 80 mm andinternal height 90 mm. This mass is determined by filling the cylinderwith wood elements in a free-flowing manner (i.e. without shaking orpushing it into the cylinder) and gently wiping off any excess woodelements (i.e. those wood elements that are over the brim) with aspatula. Then, the mass of the wood elements (“reference value”) isdetermined by weighing the filled cylinder and subtracting the emptyweight of the cylinder.

in the RWG test, the aforementioned cylinder is filled with impregnatedwood elements, in the above-described manner;

the contents of the cylinder are transferred to a beaker having adiameter of 140 mm and a height of 240 mm;

said beaker, filled with the wood elements, is placed inside a preheatednatural convection oven set at 160° C. for a period of 45 minutes;

the beaker is taken out of the oven and the mass of the wood elements(“test value”) is determined by weighing the filled cylinder andsubtracting the empty weight of the cylinder.

In the foregoing test method, the impregnated wood elements are, priorto the placement in the oven, maintained at the temperature at whichthey exited from impregnation; the exit from impregnation is to beunderstood as the exit from the final step before the wood elementswould be subjected to acetylation reaction conditions in a ReactionChamber.

The RWG (%) is calculated with formula:

[M _(test value)(g)−M _(reference value)(g)]/M_(reference value)(g)×100%

The wood elements are preferably sampled, and the RWG determined, in anymanner such that either a representative sample of the wood elements isobtained or that an average of the RWG of the batches of wood elementsis determined over the period in question.

In an alternative embodiment, the RWG is the RWG of one and the samebatch of wood elements, hereinafter indicated as RWG*. Herein the woodelements employed to determine the reference value are themselves usedfor impregnation and subjected to heating as defined above, and thenweighed so as to determine the test value for the same wood elements.

The wood elements can be the elements as indicated in the table below.It will be understood that the reference value and the test value aredetermined for the same type of wood elements. The wood elements arepreferably chips, strands, or particles, and more preferably chips.

In connection herewith, the invention also pertains to a process for theacetylation of wood elements comprising the following steps:

(a) providing wood elements impregnated with an acetylation fluidcomprising acetic anhydride and/or acetic acid so as to have a RetainedWeight Gain (RWG), as defined above, of at least 20%;

(b) introducing said impregnated wood elements into a Reaction Chamber;

(c) subjecting a continuous flow of impregnated wood elements toacetylation reaction conditions in said Reaction Chamber so as toprovide a continuous flow of acetylated wood elements exiting saidReaction Chamber.

In this process, the wood elements introduced into the Reaction Chamberpreferably have an RWG of at least 30% and more preferably of at least40%. A particular preferred feed of wood elements has an RWG of 40% to50%. It will be understood that, in this aspect, the invention is basedon the judicious choice to feed a continuous acetylation process withwood elements having a required RWG value, irrespective of the process(batch or continuous) by which they are impregnated.

As a further advantage, the invention also makes it possible to obtainhigh acetylation levels in a narrow distribution. I.e., particularly thechoice for a continuous acetylation process employing the principle ofplugflow, results in a consistent acetylation, with a relatively narrowdistribution of residence time, viewed over a population of woodelements, under acetylation conditions.

In order to obtain preferred high acetyl levels in the wood elements,the Reaction Chamber should allow a sufficient residence time and,moreover, a narrow distribution of the residence time over the elementscarried through. The latter particularly serves to provide a productthat shows a good uniformity in respect of acetyl levels as well as thelevels of residual acetic acid and acetic anhydride. Further, theReaction Chamber should preferably allow for a gentle operation aspossible, so as to optimally retain the geometry of the wood elements(particularly chips and strands) throughout the acetylation process.

The invention provides, as an advantage over typical fully continuousprocessing, that the impregnation can be conducted under conditionsoptimal for impregnation, viz. a sufficiently long standing time, withappropriate conditions of pressure and temperature. It also provides, asan advantage over processes conducted fully batchwise, the possibilityto have the acetylation reaction conducted in a more uniform manner, aswell as the general economic benefits associated with continuousproduction.

According to the invention, the overall process results in a continuousproduction of acetylated wood elements. This is enabled by collectingthe impregnated wood elements in such a manner as to provide acontinuous feed thereof to a Reaction Chamber. Several methods areavailable to this end.

One is to operate a plurality of Impregnation Chambers in parallel,ensuring that at all times, from one of these chambers, impregnated woodelements can be fed to a Reaction Chamber. This can be done, e.g., onthe basis of three such Impregnation Chambers, say chambers (A), (B),and (C). This allows chamber (A) to unload impregnated elements, whilstwood elements in chamber (B) are being impregnated, and chamber (C) isbeing loaded with elements to be impregnated. Next, chamber (B) will beunloaded, whilst the batch in chamber (C) is being impregnated, andchamber (A) is being loaded. It is preferred, in this embodiment, tooperate a larger number of Impregnation Chambers, so as to allow alonger impregnation time between loading and unloading. A preferrednumber of Impregnation Chambers is 1-15, more preferably 1-4.

In a preferred embodiment, the impregnated wood elements are notdirectly fed into a Reaction Chamber, but are collected in a CollectionChamber, preferably a funnel-type container. This can be done startingfrom the above plurality of Impregnation Chambers. However, an advantageis that the use of a Collection Chamber will allow the impregnation tobe done in a single Reaction Chamber. The only prerequisite foroperating a continuous acetylation process is that the CollectionChamber is fed with impregnated wood elements at a sufficient rate tocontinuously feed a Reaction Chamber. Advantageously, the CollectionChamber therefore has a relatively large volume, e.g. 1-15 times thevolume of the Impregnation Chamber. Optionally, the feature of aCollection Chamber is combined with the feature of a plurality ofImpregnation Chambers.

The morphology of the wood based material may be, but is not limited to,wood chips, wood strands, wood particles, etc. hereafter referred to aswood elements. The process and plant of the invention can be used forthe acetylation of durable and non-durable hardwoods, as well as durableand non-durable softwoods. The wood elements preferably belong tonon-durable wood species such as soft woods, for example, coniferoustrees, typically spruce, pine or fir, or to non-durable hardwoods.Preferred types of wood are spruce, sitka spruce, maritime pine, scotspine, radiata pine, eucalyptus, red alder, European alder, beech andbirch.

Typical dimensions of wood elements as defined in accordance with thepresent invention are given in the following table.

Typical dimensions of wood elements

length (mm) width (mm) thickness (mm) wood element from to from to fromto Chips 5 75 5 50 1.5 25 Strands 20 120 5 40 0.25 1.5 splinters(slivers) 5 75 0.15 0.5 0.15 0.5 Particles 1.5 20 0.15 5 0.15 5 Fibrebundles 1.5 25 0.15 0.5 0.15 0.5 Fibres 1 5 0.05 0.1 0.05 0.1

The benefits of the process of the invention are exhibited to thegreatest extent in the event of wood chips, strands, or particles. Mostpreferably, the wood elements are wood chips.

Wood elements of a single size range are preferred to facilitate ahomogeneous mass flow.

Before impregnation, preferably, wood elements with a moisture contentof less than 15% by weight are provided. This can be achieved bypre-drying wood elements having a higher moisture content. Thepre-drying can be done in a continuous or batch process by any methodknown in the wood industry. Preferably, the moisture content of the woodelements is less than 8%, more preferably in a range of from 0.01% to5%, still more preferably in a range of from 0.5% to 4%.

During impregnation the elements are contacted with the acetylationfluid allowing sufficient uptake of the fluid by the wood elements forsubsequent acetylation. The acetylation fluid preferably comprisesacetic anhydride and/or acetic acid, and preferably a mixture of aceticanhydride and acetic acid. In a preferred embodiment, the acetylationfluid contains 40-100 wt. % acetic anhydride and 0-60 wt. % acetic acid.Within this range a mixture of 75-95 wt % acetic anhydride and 5-25 wt %acetic acid has been found to be beneficial.

A preferred temperature range during impregnation is from 20° C. to 180°C. The elements to fluid ratio is preferably at least 1 to 4. In thecase of an anhydride/acid mixture having an excess of anhydride, apreferred temperature is 60° C. to 90° C.

Preferably, excess liquid is separated from the impregnated woodelements before these enter the Reaction Chamber. More preferably thisis realized in the aforementioned Collection Chamber. By excess is meantacetylation fluid that has not impregnated the wood. It may be removedfrom the reactor by any effective method like free draining or othertechnologies. During this step, sufficient acetylation fluid is retainedinside the elements for subsequent acetylation.

In the Reaction Chamber, the impregnated wood elements are acetylated ina reactor for continuous acetylation, according to the “first in firstout” principle. The residence time of the wood elements in theacetylation reactor can be controlled in order to attain the desiredmodification level of the elements, e.g. acetylation degree. Therequired residence time may also depend on the acetylation temperature.The acetylation temperature is 100-200° C., and preferably 135-180° C.Preferably, the acetylation step is conducted during 1 to 300 minutes,more preferably 30-180 minutes.

The acetylation in this process is preferably performed at temperatureshigher than the atmospheric boiling point of the acetylation fluid,although the overall gas pressure in the reactor is preferably aroundatmospheric. These higher temperatures significantly speed up theacetylation process. Therefore, acetylation temperatures in thiscontinuous process are preferably in the range of 135° C-180° C.,

The ratio of evaporation to reaction can be adjusted by the pressurelevel in the reaction chamber. A higher pressure at a certain reactiontemperature means a lower evaporation rate at essentially the samereaction rate. Preferably the pressure inside the reaction chamber ismaintained between −0,40 barg and 3 barg and more preferably between−0,2 barg and 1 barg and still more preferable between −0,1 and 0,5barg.

In one embodiment the “first in first out” principle is fulfilled byusing a screw conveyor filled with impregnated and separated from excessliquid wood elements. Due to the gentle rotation of the screw,preferably in a horizontal position, the wood elements are preventedfrom attrition. This results in good element quality during the process.In one embodiment the reactor contains a double, parallel screw conveyorin order to increase the effective volume of the reactor.

The temperature during the acetylation can be controlled by a heated gasloop, which contains a gas at least partially saturated with aceticanhydride and/or acetic acid or being superheated acetic acid and/oracetic anhydride. An inert gas means that it does not participate in theacetylation reaction and is preferably nitrogen, carbon dioxide, or fluegas. In another embodiment, the screw conveyor reactor comprises a screwaxle and a conveyor casing and the temperature of the acetylation iscontrolled by heating the screw axle and/or the conveyor casing. In thisembodiment, the impregnated wood elements are directly heated by thescrew axle and/or the conveyor casing. This heating can be performed bysteam, oil or electrically. In another embodiment the heating can bedone by a combination of a heated gas loop, heating by the screw axleand by the conveyor casing.

In addition to the reaction, the heating of the wood elements in thereactor also results in partial drying. Initial liquid contents of thechips, typically in the range of 100% to 200% may be reduced to 0.5% to100% during the process, without effectively hindering effectiveacetylation.

If desired, after the acetylation, the modified acetylated wood elementsmay be further dried by any method known in industry to a desired liquidcontent. Drying of the acetylated wood elements may be convenientlyeffected, e.g. by superheated vapours, hot nitrogen, carbon dioxideand/or flue gas, or vacuum.

In an interesting embodiment, the inventors have realized that suitableapparatus for the continuous acetylation of impregnated wood elementscan generally also be used for the post-drying of the acetylatedelements. Thus, e.g., one screw conveyor can be used for acetylation,and a next screw conveyor can be used for drying. In another embodimentthe reaction chamber is operated in such a way that it acts as apost-acetylation dryer as well, which would obviate the need for aseparate, dedicated post/drying unit.

In a preferred embodiment, the Reaction Chamber is a screw conveyor, andpost-drying is conducted using a so-called tray dryer. In anotherpreferred embodiment, a tray dryer can also be used as a ReactionChamber. This has, as one of its advantages, an optimally gentleoperation during acetylation. Other arrangements can be used as well.

Suitable screw conveyors include single screw and multiple screwconveyors, preferably a double, parallel screw conveyor. Such equipmentis well-known to the skilled person. Examples of suitable dryers, thatalso may be used as reaction chambers, include hot plate dryers,convective heat dryers, vacuum type plate dryers, disc dryers, traydryers, belt dryers (including multi-belt dryers, vacuum belt dryers,low temperature belt dryers), continuous rotary drum dryers, includingvacuum drum dryers, extraction trains, screw dryers.

Other equipment that is well suited as reaction chamber are immersiontype extractors and percolation type extractors.

Wood elements acetylation levels of 17% to 26% acetyl may readily beobtained by the present invention, as measured by high-pressure liquidchromatography (HPLC). The HPLC is used to quantify the acetate ionconcentration resulting from the saponification of acetyl groups.

The process of the invention, makes it possible to produce woodelements, and particularly wood chips or strands, having acetyl contentsthat before were not achievable in the art in a continuous acetylationprocess. The judicious combination of optimal impregnation in a batchprocess and optimal reaction, well distributed over the impregnatedelements, in a continuous process, achieves preferred acetylation levels(expressed as Acetyl Content) of 15% to 25%. Particularly, the processof the invention makes it possible to produce acetylated wood elementshaving acetyl levels hitherto unachievable in continuous processes, andalso unachievable in non-catalyzed acetylation processes.

In this respect, the invention also pertains to acetylated woodelements, obtainable by a non-catalyzed acetylation process, andpreferably chips, strands, or particles, wherein the acetylated woodelements have an acetyl content (AC) of at least 20.5%, preferably atleast 21%, and particularly of AC 21% to 26%.

Particularly, as a preferred form of acetylated wood elements obtainableby non-catalyzed acetylation processes, the invention pertains toacetylated wood elements, and preferably chips, strands, or particles,substantially not containing residual acetylation catalyst, wherein theacetylated wood elements have an acetyl content (AC) of at least 20.5%,preferably at least2l%, and particularly of AC 21% to 26%. Residualacetylation catalysts particularly consist of organic bases such aspyridine, salts such as sodium bicarbonate, or acetate salts, the latterincluding potassium acetate. The substantial absence of such catalystresidues contained in the acetylated wood, defines the result of anon-catalyzed acetylation process. In acetylated wood obtained by acatalyzed processes, the presence of such residues (even if at ppmscale) will normally be inevitable. It has hitherto not been possible toachieve acetylation contents of AC as mentioned above, and particularlynot of higher than 21%, more particularly of AC 21% −26% in acetylatedwood.

The presence of residual catalyst is generally found throughout thethickness of the wood element.

It is noted that the process of the invention can also be carried out asa catalyzed acetylation process, particularly if still higheracetylation contents and/or a fast reaction are desired.

Acetylated wood elements according to the present invention may usefullybe refined and converted to board, such as medium density fibreboard,MDF, or oriented strand board, OSB, or particle board, which willpossess the superior dimensional stability, durability, stability toultra-violet light and thermal conductivity, compared with board derivedfrom non-acetylated wood elements.

The invention is illustrated by the following, non-limiting examples.

EXAMPLE 1

Chips of Sitka Spruce wood with a majority of chip size approximately2.5 cm×1.5 cm×1 cm in size are dried in a convective wood drying kilndown to a moisture content of 2.4% (weight of moisture/weight of bonedry wood chips).

Before bringing this chip material into the impregnation chamber, thisimpregnation chamber is evacuated to a vacuum of −0.9 barg to removepotential residual organic fluid from the foregoing impregnation batch.After this evacuation step the dried chip material is fed into thisimpregnation vessel. Subsequently, a vacuum of −0.9 barg is implementedto this chips-filled impregnation chamber, after which the hotacetylation fluid (consisting of 90/10 w/w ratio acetic anhydride/aceticacid) at 130° C. is dosed to this evacuated impregnation vessel. Thisfilling with hot acetylation fluid is performed under evacuation action.After complete submerging of the chips with this hot acetylation fluidthe impregnation vessel is pressurized to 10 barg for 1 minute. Afterthis pressurized impregnation step the pressure is cleared toeffectively 0 barg and the material is transferred into the next processstep: in the cylindrical collection vessel the excess acetylation liquidis drained by gravitational forces. From this collection vessel step theimpregnated chips are continuously fed into the reactor chamber. Thisreaction chamber is a horizontal cylinder, tooled with a screw, to whichthe chips with the hot impregnated liquid are fed; this horizontalcylinder is equipped with a heating gas loop and a condensing system tomaintain the reaction pressure at effectively 0 barg. The heating gasthat is passing the reacting impregnated chips comprises a mixture ofacetic anhydride and acetic acid. The heating gas loop temperature iscontrolled at 135° C.; this acetylation condition is continuouslyoperated at effectively 0 barg. After a residence time of 225 minutes inthis reaction vessel the chips are highly acetylated where the acetylcontent of the chips amounts to 22.5% as measured by HPLC. After thisacetylation reaction step the acetylated chips are dried at 130° C.under reduced pressure to remove combined residual acetic anhydride andresidual acetic acid to levels below 0.4%.

EXAMPLE 2

Wood particles of Sitka Spruce with a majority of particle sizeapproximately 1 cm×0.5 cm×0.5 cm in size are dried in a convective wooddrying kiln down to a moisture content of 3% (weight of moisture/weightof bone dry wood particles).

The dried particles are fed into the impregnation vessel, which is thenevacuated to −0.9 barg; after this evacuation the hot acetylation fluid(consisting of 90/10 w/w ratio acetic anhydride/acetic acid) with atemperature of 70° C. is dosed to this evacuated impregnation vessel.This filling with hot acetylation fluid is performed under continuousvacuum. After complete submerging of the particles with this hotacetylation fluid the impregnation vessel is pressurized to 10 barg for1 minute. After this pressurized impregnation step the pressure iscleared to effectively 0 barg and the material is transferred into thenext process step: in the cylindrical collection vessel the excessacetylation liquid is drained by gravitational forces. From thiscollection vessel step the impregnated wood particles are continuouslyfed into the reactor chamber. This reaction chamber is a horizontalcylinder, tooled with a screw, to which the particles with the hotimpregnated liquid are fed; this horizontal cylinder is equipped with aheating gas loop and a condensing system to maintain the reactionpressure. The heating gas that is passing the impregnated particlescomprises a mixture of acetic anhydride and acetic acid. The heating gasloop temperature is controlled at 135° C.; this acetylation condition iscontinuously operated at effectively 1 barg. After a residence time of120 minutes in this reaction vessel the wood particles are acetylated to21.9% as measured by HPLC. After this acetylation reaction step theacetylated chips are dried at 130° C. under reduced pressure to removecombined residual acetic anhydride and residual acetic acid to levelsbelow 0.4%.

EXAMPLE 3

In this example, strands of Southern Yellow Pine with a majority ofstrand size approximately 10 cm×4 cm×0.1 cm in size are dried in aconvective wood drying kiln down to a moisture content of 2%.

This dried strand material is fed into the impregnation vessel; afterfilling the above dried strands a vacuum of −0.95 barg is implementedfor 10 minutes to this strands-filled impregnation chamber, after whichhot acetylation fluid (consisting of 90/10 w/w ratio aceticanhydride/acetic acid) at 70° C. is dosed to this evacuated impregnationvessel. This dosing is performed under constant vacuuming. The dosing ofthe hot acetylation fluid is carried until complete submerging of thestrands with this liquid. After complete submerging of the strands withthis hot acetylation fluid the impregnation vessel is pressurized to 10barg for 10 minutes. Subsequently the impregnation vessel is evacuatedand again pressurized to 10 barg for another 10 minutes. After thesealternating impregnation steps the pressure between the impregnationvessel and the collection chamber is levelled, because the collectionchamber is operated at effectively 2 barg. After opening the bottomvalve of the impregnation vessel the batch of impregnated strandstogether with the excess liquid is dropped into the collection chamber.Transport of the impregnated strands with an upward tilted transportscrew allows for separation of the strands from the excess acetylationfluid. These drained impregnated strands that are fed to the reactionvessel contain about 1.6-1.8 kg of acetylation liquid per every kg ofdry wood strands. After this separation from the excess acetylationliquid the strands are introduced in the reaction vessel, which consistsof a heated horizontal double screw that rotates such that the residencetime of the strands in this heated horizontal double screw is 120minutes. This horizontal double screw is equipped with a heated gasloop. The heating gas that is passing the reacting impregnated strandscomprises a mixture of acetic anhydride and acetic acid. The heating gasloop temperature is controlled at 135° C.; this acetylation condition iscontinuously operated at effectively 2 barg. After a residence time of120 minutes in this reaction vessel the acetyl content of the strandsamounts to 21.2±0.1%. After this acetylation reaction step theacetylated strands are dried at 130° C. under reduced pressure to removecombined residual acetic anhydride and residual acetic acid to levelsbelow 0.9%.

1. A process for the acetylation of wood elements comprising thefollowing steps: (a) impregnating a plurality of batches of woodelements with an acetylation fluid comprising acetic anhydride and/oracetic acid so as to provide a plurality of batches of impregnated woodelements; (b) collecting said plurality of batches of impregnated woodelements in such a manner as to provide a continuous feed of impregnatedwood elements to a Reaction Chamber; (c) subjecting a continuous flow ofimpregnated wood elements to acetylation reaction conditions in saidReaction Chamber so as to provide a continuous flow of acetylated woodelements exiting said Reaction Chamber.
 2. The process according toclaim 1, wherein collecting the batches of acetylated wood elements isdone in a Collection Chamber, preferably having a funnel-type geometry.3 The process according to claim 2, wherein excess impregnation liquidis removed from the impregnated wood elements in the Collection Chamber.4. The process for the continuous acetylation of wood elementscomprising the following steps: (a) providing wood elements impregnatedwith an acetylation fluid comprising acetic anhydride and/or acetic acidso as to have a Retained Weight Gain (RWG) of at least 20%, RWG beingdefined as[M_(test value)(g)−M_(reference value)(g)]/M_(reference value)(g)×100%wherein M_(reference value) is the mass of a volume of wood elementscontained in a glass value is cylinder, of known empty weight, internaldiameter 80 mm and internal height 90 mm, said mass being determined byfilling the cylinder with wood elements in a free-flowing manner andwiping off any excess wood elements with a spatula, weighing the filledcylinder and subtracting the empty weight of the cylinder, and whereinM_(test value) is determined by filling the aforementioned cylinder withimpregnated wood elements in a free-flowing manner and wiping off anyexcess wood elements with a spatula, transferring the contents of thecylinder to a beaker having a diameter of 140 mm and a height of 240 mm;placing said beaker, filled with the wood elements, inside a preheatednatural convection oven set at 160° C. for a period of 45 minutes;taking the beaker out of the oven and determining the mass of the woodelements by weighing the filled cylinder and subtracting the emptyweight of the cylinder; (b) introducing said impregnated wood elementsinto a Reaction Chamber and subjecting said impregnated wood elements toacetylation reaction conditions in said Reaction Chamber; (c)withdrawing a continuous flow of acetylated wood elements from saidReaction Chamber.
 5. The process according to claim 1, wherein theimpregnated wood elements are continuously introduced into the ReactionChamber.
 6. The process according to claim 1, wherein the wood elementsflow through the Reaction Chamber in plug flow.
 7. The process accordingto claim 1, further comprising a step: (d) drying the acetylated woodelements so as to remove unreacted acetylation fluid therefrom.
 8. Theprocess according to claim 7, further comprising a step (e) withdrawinga continuous flow of acetylated wood elements from the drying apparatusof step (d).
 9. The process according to claim 1, wherein the ReactionChamber comprises a horizontal screw conveyor, such as a double,parallel screw conveyor.
 10. The process according to claim 1, whereinthe moisture content of the wood elements subjected to step (a) is lessthan 8% by weight, preferably 0.01% to 5%.
 11. The process according toclaim 1, wherein the acetylation fluid comprises 40% by weight to 100%by weight acetic anhydride and 0% by weight to 60% by weight aceticacid.
 12. The process according to claim 1, wherein the acetylationtemperature in step (c) is higher than the atmospheric boiling point ofthe acetylation fluid.
 13. The process according to claim 1, wherein theacetylation in step (c) is performed in two to five temperature zones.14. The process according to claim 1, wherein the temperature ofacetylation in step (c) is controlled by a heating an environmentselected from a heated gas loop containing a gas at least partiallysaturated with acetic anhydride and/or acetic acid, a heated mantle, aheated axis, and combinations thereof.
 15. A plant for the acetylationof wood elements, said plant comprising (i) at least one ImpregnationChamber, (ii) a Collection Chamber having an inlet in connection with anoutlet of the at least one Impregnation Chamber, said connection beingsuch as to allow wood elements to flow from the Impregnation Chamber tothe Collection Chamber; (iii) a Reaction Chamber having an inlet inconnection with the outlet of the Collection Chamber, said connectionbeing such as to allow wood elements to flow from for the CollectionChamber to the Reaction Chamber, wherein the Reaction Chamber isdesigned so as to provide a continuous transport of wood elements fromthe inlet to an outlet thereof.
 16. The plant according to claim 15,wherein the continuous transport in the Reaction Chamber is provided bya screw conveyor.
 17. The plant according to claim 15, wherein theReaction Chamber is a dryer, preferably selected from the groupconsisting of tray dryers, belt dryers, extraction trains, screw dryersand plate dryers.
 18. The plant according to claim 15, furthercomprising a dryer, preferably a tray dryer, the inlet of which is inconnection with the outlet of the Reaction Chamber, said connectionbeing such as to allow wood elements to flow from the Reaction Chamberto the Dryer.
 19. Acetylated wood elements, obtainable by anon-catalyzed acetylation process, and preferably chips, strands,slivers, or particles, wherein the acetylated wood elements have anacetyl content (AC) of at least 20.5%, and preferably of AC 21% to 26%.